Rspamc can internally process directories and process multiple files
simultaneously (using -n option, 10 is the default). A single message
can be processed for a longer time since Rspamd performs a lot of DNS
and other network checks which could be very slow (but that's all about
scan latency not about scan throughput).
In general, Rspamd is designed to process multiple messages at the same
time and not one message by another.